1. Field of the Invention
The present invention relates in general to rotors for use in turbomachines, such as displacement type vacuum pumps or compressors operating in dry state, i.e., without using lubricant in the fluid passages, and relates also to a method of manufacturing such rotors.
2. Description of the Related Art
There are turbomachines of displacement type vacuum pumps and compressors comprising a pair of rotors, each of which has lobes with an involute or cycloid peripheral profile. The rotors are synchronously rotated so that the lobes of each rotor are engaged with that of the other rotor thereby for pressurizing and transporting fluid.
Conventional rotors generally have been made as a unitary solid part. Those solid rotors are manufactured, for example, by casting the rotor and rotation shafts as an integral unit, or by an insert casting around the rotation shaft, or by mechanically fixing a solid rotor to a rotation shaft by a keyed arrangement.
However, such solid rotors are heavy to result in inefficiencies in fabrication and assembly operations, as well as a high material cost. Further, since these rotors have a high inertial moment, they cannot be accelerated or decelerated quickly during the startup or stopping operation. Other problems relate to the possibility of damaging the casing should the rotating rotor fail by fracture, and to the difficulty in dynamic balancing because of non-uniformness of the surface of the rotor.
Techniques to produce hollow rotors have been developed comprising lamination process of punched sheet metals, but this approach presents a productivity problem because of the difficulty of bonding of the laminates, and the cost of assembly tends to be high. For this reason, there has also been a suggestion to produce hollow rotors by making hollow sections inside the rotors, as disclosed in Japanese Patent Application, First Publication H7-151082, for example.
However, the approach disclosed in Japanese Patent Application First Publication No. H7-151082 presents a limitation in reducing the weight, because the rotors are made by casting which creates a limit on the wall thickness reduction achievable. The approach also presents another problem regarding uniformness of the wall thickness because of the limitations inherent in the casting technique.
Therefore, there has been a need to provide light weight and low inertial rotors for use in turbomachines, which can be produced efficiently and at low cost, and an accompanying need for a new method of production of such rotors.